The thermodynamics of myoglobin stability. Effects of axial ligand.

The reversible thermal denaturation of four metmyoglobin derivatives, aquomet, cyanomet, azidomet and fluoromet in both the alkaline pH and acidic pH region, has been examined by optical spectrophotometry. The data are analyzed in terms of a two-state model to extract the thermodynamic parameters delta G, delta H, delta S and delta Cp characterizing the transition. The results are consistent with Brandts' phenomenological model of protein denaturation. Among the derivatives examined, cyanomet, azidomet and fluoromet are about 3 kcal/mol, 2 kcal/mol and 0.2 kcal/mol, respectively, more stable than aquometmyoglobin. The observed differences are found to be inconsistent with the hypothesis that the stability is mainly governed by the spin state of the iron atom. In addition, the enthalpic and entropic contributions in delta G are extracted and the differences in delta G for the various derivatives are found to arise from minor changes in delta H and T delta S. Assuming the final denaturated state to be universal, these changes reflect the effect of ligands on the conformational energy of the native protein.